The presence of large deposits of oil shale in the semiarid high plateau region of the Western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in formations containing oil shale. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer, it is neither shale nor does it contain oil. It is a sedimentary formation comprising a marlstone deposit with layers containing an organic polymer called "kerogen" which upon heating thermally decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid product produced by the decomposition of the kerogen is called "shale oil."
A number of methods have been proposed for processing oil shale which involved either first mining the oil shale and processing the oil shale above ground, or processing the oil shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirements for disposal of solid waste.
Many of the methods for shale oil production are described in Synthetic Fuels Data Handbook, 2nd ed., compiled by Dr. Thomas A. Hendrickson, and published by Cameron Engineers, Inc., Denver Colo. Above ground retorting processes include those known as Tosco II, Paraho direct, Paraho indirect, N-T-U, and Bureau of Mines, Rock Spring processes.
The Tosco II retorting process is described in pp. 85-88 of the Synthetic Fuels Data Handbook and U.S. Pat. No. 3,025,223. Briefly, this process involves preheating minus one-half inch oil shale particles to about 500.degree. F. in an entrained bed lift pipe. The preheated oil shale particles are then introduced to a rotating pyrolysis drum. The heat for retorting the oil shale particles is provided by heated ceramic balls which are separately heated in a ball heating furnace and introduced to the rotating drum. The ceramic balls are heated to a temperature of about 1200.degree. F. and are provided to the pyrolysis drum in an amount sufficient to heat the oil shale particles to about 900.degree. F.
The Paraho process is described at pp. 100-104 of the Synthetic Fuels Data Handbook and the U.S. patents referred to therein. The Paraho process employs a vertical kiln through which ground oil shale moves downwardly as gas moves upwardly. Combustion air can be admitted into the bed of oil shale particles for direct heating of the oil shale by combustion within the bed. This process is referred to as Paraho direct. The kiln can also be arranged so that recycled gas can be heated externally, then injected into the bed of oil shale for indirect heating of the oil shale. Such a process is referred to as the Paraho indirect process. The N-T-U process is a batch process which is described at pp. 67-72 of the Synthetic Fuels Data Handbook and the United States patents referred to therein. In the N-T-U process, a retort is filled with a batch of oil shale particles and ignited at the top. Combustion is supported by air injection and a combustion zone is passed downwardly through the stationary bed of oil shale particles. Recycled gas from the bottom of the retort is mixed with the combustion gas to modulate temperatures and provide some of the fuel requirement. Other above-ground oil shale retorting processes described in the Synthetic Fuels Data Handbook include the gas combustion process on p. 72; the Kiviter process on p. 76; the Petrosix process described on p. 80; the Lurgi-Ruhrgas process described on p. 81; Superior Oil process described on p. 88; the Galoter process described on p. 90; the Institute of Gas Technology process using hydrogen retorting described on p. 92; and the Union Oil process described on p. 95.
Various in situ oil shale retorting processes are disclosed. Beginning on p. 104 of the Synthetic Fuels Data Handbook.
The Bureau of Mines, Rock Springs process is described in Paper No. SPE-6067, by R. L. Wise et al, prepared for the 51st annual technical conference and exhibition of the Society of Petroleum Engineers of AIME, held in New Orleans, Oct. 3 to the 6th, 1976. Such a process is also described in U.S. Pat. No. 3,346,044, among others. Generally, this process involves fracturing of an underground oil shale formation and propping the fractions open with sand. Injection and production wells are drilled into the fractured formation. A combustion zone is moved from an injection well towards one or more production wells for retorting oil shale in the fractured formation.
The liquid oil product recovered from the retorting processes and which is the result of the thermal decomposition of the organic material, kerogen, in the oil shale is referred to as shale oil. The properties of the crude shale oil product from a retorting process are dependent on a variety of factors which occur during the retorting process. One of the most important of these factors is temperature, or more specifically, temperature history. Not only is the retorting temperature important, but the rate at which the oil shale was heated to this temperature and the time at which it is kept at the retorting temperature are of concern. The temperature to which the shale oil vapors are heated after they are generated is a factor affecting the properties of shale oil, as well as the length of time such vapors are exposed to this temperature.
Generally, the crude shale oil tends to thicken when cooled and progressively becomes increasingly resistant to flow in the fluid handling operations, such as pumping through a pipeline. However, there is little or no relation between the viscosity and the pour point of a particular shale oil. The temperature at which the shale oil changes from a flowable to a nonflowable state, as measured by ASTM D97, is called the pour point. At temperatures from slightly above the pour point to below the pour point, the shale oil can be difficult or impossible to pump, requiring the use of costly heated pipelines, tank cars and the like. The transportation of such shale oil is thus hindered, particularly in colder months. Because shale oil is produced from oil shale deposits located far from population centers and refining facilities in areas of the Western United States subject to severe winters, practical methods for regulating the pour point of shale oil are needed.
The pour point of shale oil preferably should be low enough to allow the oil to be pumped through pipelines. Higher pour points are acceptable in warmer climates or warmer months of the year, and conversely lower pour points are required when cooler temperatures prevail. It is considered that, in the Piceance Creek Basin of Western Colorado during the winter months, shale oil having a pour point lower than 20.degree. F. can generally be pumped satisfactorily, even though prevailing temperatures can be much below 20.degree. F. This is because the shale is warm, e.g., above about 100.degree. F. when it is withdrawn from an in situ oil shale retort. The warm oil can be pumped and once it is flowing it can continue to flow when its temperature drops below its pour point. However, if the flow of oil is interrupted, it can set up to an unpumpable state if cooled below its pour point and warming can be required before pumping can be resumed.
One method of lowering the pour point of high pour point shale oils which may be exposed to low temperatures is to add a pour point depressant to the shale oil. A pour point depressant can be any additive that is effective for lowering the pour point of the shale oil. Various pour-point depressants are known and have been used successfully, mostly with middle distillate fuels. Some of the known pour-point depressants can be expensive and some may need to be used in such large amounts that they undesirably effect the shale oil product. For example, pour-point depressants may need to be removed prior to further refining or processing of the shale oil. A further difficulty with pour-point depressants is that the influence on the pour point of a shale oil by any particular substance is unpredictable. This unpredictability is generally thought to be due to structural differences of the paraffins occurring in the various shale oils which differences are derived from different sources or locations and retorting methods.
Various pour-point depressants are disclosed in the art. For example, as disclosed hereinafter, various pour-point depressants can be made from shale oil and other sources. Use of particular polymers as pour-point depressants for residium-containing oils and heavy petroleum fractions is known for example, see U.S. Pat. Nos. 3,567,639 and 3,817,866.
U.S. Pat. No. 3,523,071 to Knapp et al teaches that the heavy fraction produced from visbreaking raw shale oil is an effective pour-point depressant by hydrodenitrogenated shale oil.
U.S. Pat. No. 3,532,618 of Wunderlich et al discloses hydrovisbreaking shale oil and deasphalting the visbroken shale oil to produce a deasphalted shale oil of intermediate pour point and an asphaltine portion which can have utility as a pour-point depressant for shale oil.
U.S. Pat. No. 3,369,992 discloses converting a high wax, high pour point oil into a low pour point synthetic crude oil by separating the high pour point oil into a virgin distilate and a reduced crude, coking the reduced crude and combining a middle fraction of the coker distillate with the virgin distillate to produce a low pour point product.
U.S. Pat. No. 4,029,571 discloses reducing the pour point of a synthetic crude oil by hydrovisbreaking or visbreaking the oil. The hydrovisbroken or visbroken oil exhibits a reduced pour point from that of the crude oil.
U.S. Pat. No. 3,738,931 discloses hydrovisbreaking shale oil, separating and hydrogenating the visbroken vapors and combining them with the visbroken liquid to produce a shale oil having a reduced pour point.
U.S. Pat. No. 3,284,336 to Culbertson, Jr. et al discloses separating shale oil into heavy and light fractions, thermally treating only the heavy fraction at a temperature from 600.degree. F. to below the point of thermal decomposition, and recombining the thermally treated fraction with the light fraction to give a product having a reduced pour point.
U.S. Pat. No. 4,201,658 to Jensen discloses that a pour-point depressant can be formed by thermally treating a raw whole shale oil in substantially liquid phase at a temperature from 600.degree. F. to below the point of significant thermal decomposition to form a thermally treated shale oil. The thermally treated shale oil is then deasphalted by mixing with a deasphalting solvent. The insoluable asphaltine component has utility as a pour-point depressant.
U.S. Pat. No. 4,181,177 of Compton, discloses that a blended shale oil composition can be formed which has a pour point different from a crude shale oil. The process of changing the pour point of crude shale oil as disclosed in the patent is practiced on a crude shale oil produced by in situ retorting of oil shale. The crude shale oil obtained from the in situ retort is fractionated to produce either a low boiling fraction, a paraffinic fraction, or a high boiling fraction having a relatively higher paraffin content than the overall paraffin content of the shale oil. Such fractions of the crude shale oil can each be blended with crude shale oil to provide a blended shale oil composition having a pour point within the range for convenient handling.
As disclosed in the Synthetic Fuels Data Handbook, the pour point of crude shale oil produced by above ground retorting of oil shale is relatively high, i.e., about 60-90.degree. F. In Table 75 on p. 115 of the Synthetic Fuels Data Handbook, the processes for producing shale oil and the pour point of the shale oil produced by such processes are given as follows: N-T-U, 80.degree.; N-T-U, 90.degree.; N-T-U, 70.degree.; gas combustion, 83.5.degree.; gas combustion, 85.degree.; Tosco II, 80.degree.; Union, 80.degree.; Union, "V", 60.degree.; Paraho, 85.degree.; Hydrotort, 65.degree.; and Catalytic Hydrotort, 75.degree..
It would be desirable to have a method for reducing the pour point of such above ground process-produced shale oils to provide a pipelineable and storable shale oil.